An electrophotographic photoreceptor has an electrically conductive support and a photosensitive layer formed thereon which includes an inorganic or organic photoconductor.
Recently, double layer photoreceptors with a combination of a charge-generation layer and a charge-transport layer have been shown to have higher sensitivity and some of them have been commercially employed. Those photoreceptors in which an organic material is used as the charge-transporting medium and which have high charge acceptance in addition to higher sensitivity have been highly investigated.
An electrophotographic photoreceptor is subjected to repeated copying operation which include charging by a corona charging device, exposing, developing, transferring and cleaning steps and is required to have excellent durability in such repeated copying operations. When the above-mentioned prior photoreceptors, especially those photoreceptors having an organic charge-transporting medium are subjected to the repeated copying operation (for example, several thousands to several ten thousands times), however, both abrasion and cracks are observed in the photoreceptor due to practical loads such as development with the toner, friction with paper and/or cleaning means and therefore the printing-resistance is actually limited.
The above-mentioned phenomena are mainly caused by the low surface strength of the charge-transport layer. One attempt at increasing the surface strength of the charge-transport layer, i.e., the selection of a suitable polymeric binder (which is generally included together with a charge-transporting material in the charge-transport layer) resulted in failure since a large amount of the charge-transporting material was doped therein.
A method for providing a protective layer on the charge-transport layer to improve the surface strength of the photoreceptor has also been proposed. In this method, the protective layer is formed by coating the charge-transport layer with a solution in which a thermo-setting silicone resin is dissolved and then setting the resin by heating. However, this protective layer has problems such as the occurance of cracks and cuts as well as an ease of separation from the charge-transport layer since the silicone resin protective layer could unsatisfactorily adhere to the charge-transport layer. Further, the protective layer may partially peel off the charge-transport layer due to the pressure of the cleaning means and the like.
A method for providing an adhesive layer between the protective layer and the charge-transport layer to improve the adhesive strength therebetween has been also proposed. This method is not practical since it offers undesirable effects such as increase in the residual potential and the development fog by the presence of the adhesive layer.
Thus, there is a strong need for increasing the adhesive strength between the protective layer and the charge-transport layer without any of the above-described undesirable effects that can provide the electrophotographic photoreceptor with improved durability for repeated copying operations.
An object of this invention is to provide such an electrophotographic photoreceptor having improved durability in repeated copying operations by improving the adhesive strength between the protective layer and the charge-transport layer.